STEL stands for short-term exposure limit. This is a term used to describe the amount of exposure to a specific substance that is acceptable over a short period of time. Unless otherwise stated, the STEL of something can be assumed to be over a period of 15 minutes at a time. In most cases, the STEL will apply to specific chemicals, gasses, or other substances that people work with on a regular basis.
There are many different types of substances that can be harmful to people if they are exposed to them for too long. For example, VOC chemicals like butadiene, benzene, ethylene oxide, and many others. These chemicals can build up in the body and become harmful over time. Of course, having no exposure to these chemicals is the ideal, but that is not always possible. Understanding how long someone can be exposed to them before they experience any harmful effects is critical.
The STEL is typically going to be measured in parts per million, or PPM. This means that someone can be exposed to a specific chemical at a specific concentration (PPM) for 15 minutes before any lasting harm is caused. This assumes that there is no personal protective equipment being used. If someone has a self-contained respirator, for example, the STEL rating would not apply since they would be breathing fresh air while being exposed.
There are many agencies and groups that publish STEL ratings for various chemicals and substances. The most famous one is the Occupational Safety and Health Administration, or OSHA, in the United States. The American Conference of Governmental Industrial Hygienists also publishes an extensive list of threshold limit values for chemicals that can be very helpful.
If your facility works with any type of potentially harmful substances, make sure to know the short-term exposure limit of them so you can be sure that your employees are kept safe.